Foaming during sludge treatment has traditionally been associated with activated sludge systems which have been victim to this phenomenon for several decades. However, foaming has been documented in anaerobic digestion plants, leading, fundamentally, to a reduction in digester capacity and an increase in operational expenditure. The requirements for foaming of: surface active agents, gaseous phase and hydrophobic material are all present in the anaerobic digester. Although there are numerous causes for digester foaming, a great deal of work has concentrated on the effects of GALO (previously NALO – Nocardia Amarae Like Organisms) present in secondary sludge entering a digester, on subsequent foaming. It is expected that due to external pressures encouraging biological nutrient removal, digester foaming will become an increasingly serious issue as more digestion systems will switch to 100% secondary sludge treatment (as primary is used as carbon source for phosphorous removal and/or denitrification), and plants upgraded to nitrification unwillingly encourage the growth of the scavenging filamentous organisms responsible for foaming. This paper looks at the issues involved, theory behind foaming, and potential ways to combat foaming during anaerobic digestion.

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